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Bumps and knobs occupy this tiny section on the floor of Palos Crater. Scientists think this terrain is weathering into polygon-shaped blocks. Winds brought in the dark sand and dust that fills the eroded, circular impact craters.

Sand dunes line the valley floor in this 0.6-mile-wide (1 kilometer) section of Samara Valles. This entire ancient valley system runs more than 620 miles (1,000 km) across the heavily cratered southern highlands before emptying into Chryse Basin.

Martian gullies occur on steep slopes, most often on the walls of craters at middle and high latitudes. Typically, a gully shows a branching pattern at its head and a fan-shaped debris apron at its base. Some scientists think liquid water carves the gullies, while others believe they form as chunks of frozen carbon dioxide roll downhill.

HiRISE discovered “recurring slope lineae” — the dark, narrow streaks that appear to flow down steep slopes and grow, fade, and reappear every martian year — that some scientists think could be seasonal flows of briny water. This HiWish image shows several originating in the boulder-strewn terrain at left.

The edge of a single mesa snakes through this view of Aureum Chaos, a broad region of plateaus and mesas near the equator. The rocks here likely formed as layers of sediments or volcanic debris. The ground later collapsed to create the current landscape.

The sand dunes at the base of this huge mesa might make a good dirt track course in the distant future. Racers would have to navigate the 8.7-mile (14 km) course under a gravitational pull barely one-third that at Earth’s surface.

Layers of dusty ice up to 2 miles (3 km) thick and about 620 miles (1,000 km) in diameter form the north polar layered deposits. The exposed layers seen here have various surface textures, hinting that the underlying layers have a different dust content or ice-grain size. The angular breaks mark where layers eroded before new ones were deposited.

Have you ever found a needle in a haystack? Then you might be ready to search HiRISE images for artifacts from defunct spacecraft. A team of Russian space enthusiasts used a HiWish request to locate what appears to be the parachute from the 1971 Mars 3 lander. The original image measures 3.1 miles (5 km) across; the suspected white parachute (inset, center) is just 25 feet (7.5 m) in diameter.

These concentric cracks mark a crater that formed when the surface collapsed. Scientists think a mudflow covered the original landscape, which was rich in water or water ice. A nearby source of magma then either quickly turned the water to steam or melted the subsurface ice, triggering the collapse.

This close-up shows flows of ice-rich debris along the edge of a mesa near the boundary between martian lowland and highland terrain. Scientists think material deposited only about 10 million years ago covers much of the area.

The distinctive shapes of these sand dunes in Hellas Basin arise because the wind blows in the same direction (east to west) for long periods. By imaging regions like this over many years, scientists can measure how fast the dunes creep across the surface.